Noise cancellor

ABSTRACT

If a mode calculating section ( 52 ) judges that an inputted modulated signal is nearly in a no-sound state, the interpolation width determined by an interpolation width calculating section ( 53 ) is increased, whereas if the mode calculating section ( 52 ) judges that the inputted modulated signal contains a lot of high-frequency components, the interpolation width determined by the interpolation width calculating section ( 53 ) is decreased.

TECHNICAL FIELD

[0001] The present invention relates to a noise canceller for cancelingnoise contained in an inputted signal, and more particularly to a noisecanceller as incorporated in an FM receiver apparatus to cancel noisecontained in an FM reception signal.

BACKGROUND ART

[0002] In a car-mounted FM receiver apparatus, the received FM receptionsignal has pulse noise such as ignition noise superimposed thereon, andtherefore, for the purpose of canceling such pulse noise contained inthe FM reception signal, there is provided a noise canceller.

[0003] With a conventional noise canceller, when a composite signalhaving pulse noise superimposed thereon as shown in FIG. 6A is received,the pulse noise is detected by passing the composite signal through aHPF (high-pass filter). When the HPF detects the pulse noise, a pulsenoise detection signal as shown in FIG. 6B is produced. When the pulsenoise detection signal is fed to an integrator, the integrator yields anoutput as shown in FIG. 6C.

[0004] Specifically, when the integrator is fed with the pulse noisedetection signal, a capacitor included in the integrator is charged,making the output of the integrator higher than a threshold value. Whenthe output of the integrator becomes higher than a predeterminedthreshold value in this way, the integrator is so controlled that thecapacitor is discharged, causing the output of the integrator todecrease gradually. By comparing the output of this integrator and thepredetermined threshold value, a gate control signal is produced. Withthis gate control signal, the operation of a gate circuit for cancelingthe pulse noise is controlled.

[0005] Thus, when the output of the integrator is as shown in FIG. 6C, apulse noise detection signal is produced, and while the output of theintegrator remains higher than the threshold value, the gate controlsignal remains high. The gate circuit performs signal processing suchthat the signal level of the composite signal is held at its signallevel immediately before the occurrence of the pulse noise. As a result,the composite signals is output after having the pulse noise cancelledtherefrom as shown in FIG. 6D.

[0006] However, in a noise canceller that cancels pulse noise byoperating as shown in FIGS. 6A to 6D, during the occurrence of pulsenoise, the gate circuit maintains the signal level immediately beforethe occurrence of pulse noise irrespective of the state of the receptionsignal. This causes distortion in the reception signal, resulting inunsatisfactory quality of the sound reproduced therefrom.

[0007] Incidentally, Japanese Patent Application Laid-Open No. H8-56168proposes an FM receiver apparatus that switches filters according to thestate of reception, wherein the gate period during which the signallevel of the reception signal is maintained to cancel pulse noise isvaried so as to achieve appropriate cancellation of the pulse noise.This method, however, is no different from the operations illustrated inFIGS. 6A to 6D in that the pulse noise is cancelled by maintaining thesignal level immediately before the occurrence thereof, causingdistortion in the reception signal.

DISCLOSURE OF THE INVENTION

[0008] An object of the present invention is to provide a noisecanceller that performs interpolation according to the state of thereception signal after canceling pulse noise therefrom.

[0009] To achieve the above object, according to the present invention,a noise canceller that includes a pulse position determining section fordetecting pulse noise superimposed on an audio signal and that cancelsfrom the input signal the pulse noise detected by the pulse positiondetermining section is provided with: a state calculating section thatevaluates the state of the audio signal; an interpolation widthcalculating section that, according to the proportion of high-frequencycomponents contained in the audio signal as evaluated by the statecalculating section, sets an interpolation width at the data position atwhich to cancel the pulse noise and perform interpolation; and a pulsenoise reducing section that processes the data present within theinterpolation width set by the interpolation width calculating section,with the center of the interpolation width located at the data positionat which the pulse position determining section has detected the pulsenoise from the audio signal, so as to cancel the pulse noise and performinterpolation and that then outputs the audio signal thus processed.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a block diagram showing the internal configuration of anFM receiver apparatus incorporating a noise canceller according to theinvention.

[0011]FIG. 2 is a block diagram showing the internal configuration ofthe noise canceller according to the invention.

[0012]FIG. 3 is a diagram showing the operation performed to achievecorrection.

[0013]FIGS. 4A to 4C are diagrams showing how a modulation signal in ano-sound state is corrected.

[0014]FIGS. 5A to 5C are diagrams showing how a modulation signal of asinusoidal wave having a frequency of 3 kHz is corrected.

[0015]FIGS. 6A to 6D are diagrams showing various signals illustratingthe operation of a conventional noise canceller.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will bedescribed with reference to the drawings. FIG. 1 is a block diagramshowing the internal configuration of an FM receiver apparatusincorporating a noise canceller according to the invention. FIG. 2 is ablock diagram showing the internal configuration of the noise cancelleraccording to the invention.

[0017] The FM receiver apparatus shown in FIG. 1 includes: an antenna 1for receiving broadcast signals; a front-end section (FE) 2 forselecting, from among the broadcast signals received by the antenna 1,the FM reception signal of a desired channel and subjecting it to RFamplification; an intermediate frequency amplifier section (IF) 3 forconverting the FM reception signal selected by the FE 2 to anintermediate frequency of 10.7 MHz and amplifying it; a detector section4 for extracting a modulation signal by detecting the FM receptionsignal that have been subjected to frequency conversion by the IF 3; anoise canceller (NC) 5 for canceling noise superimposed on themodulation signal obtained through detection by the detector section 4;a multiplexer (MPX) 6 for separating the modulation signal having noisecancelled therefrom by the NC 5 into audio signals to be fed to a leftand a right speaker 7 and 8; and a left and a right speaker 7 and 8 fromwhich to reproduce sound.

[0018] When the FM reception signal of a desired channel frequency isselected by the FE 2 from among the broadcast signals received by theantenna 1, then, in the IF 3, the selected FM reception signal is mixedwith a local oscillation signal and is thereby converted to anintermediate frequency. Then, in the detection section 4, the FMreception signal thus converted into an intermediate frequency isdetected by a detection method such as one based on a phase-locked loopto obtain a modulation signal. Moreover, in the detection section 4, themodulation signal is converted into a digital signal. This modulationsignal is then fed to the NC 5, where the noise superimposed on themodulation signal is detected and cancelled. The modulation signal thushaving noise cancelled therefrom is then fed to the MPX 6, whichprocesses the main and sub channel signals contained in the modulationsignal to separate it into audio signals to be fed to the left and rightspeakers 7 and 8, and then feeds those audio signals to the left andright speakers 7 and 8.

[0019] Now, the NC 5 incorporated in this FM receiver apparatus will bedescribed. The NC 5 shown in FIG. 2 includes a pulse positiondetermining section 51 for detecting pulse noise superimposed on themodulation signal obtained from the detection section 4; a modecalculating section 52 for evaluating the state of the modulationsignal; an interpolation width calculating section 53 for setting,according to the state of the modulation signal as evaluated by the modecalculating section 52, an interpolation width within which to performinterpolation around the position at which the pulse noise has beendetected; and a pulse noise reducing section 54 for canceling the pulsenoise detected by the pulse position determining section 51 andperforming interpolation after cancellation of the pulse noise.

[0020] When the modulation signal in the form of discrete digitalsignals is fed to the NC 5, the pulse position determining section 51detects the position at which the pulse noise is superimposed on themodulation signal. Here, for example, the modulation signal is firstfiltered with a high-pass filter, and is then formed into an absolutevalue with an absolute value circuit. The modulation signal thus formedinto an absolute value is then passed through a limiter circuit so thata portion thereof with an extremely large amplitude is removedtherefrom, and is then fed to a time-average circuit to calculate thetime average. Then, the signal level of the modulation signal formedinto an absolute value is compared with the time average, and, if thesignal level is sufficiently high relative to the time average, pulsenoise is recognized to be occurring, and its position is detected.

[0021] Incidentally, the inventor of the present invention proposed thedetail of the pulse position determining section 51, for example, inJapanese Patent Application Laid-Open No. 2001-102944, titled “NoiseDetection Apparatus in a Radio Receiver.” In the embodiment underdiscussion, the pulse position determining section is assumed to bebased on the noise detection apparatus proposed in the Japanese PatentApplication Laid-Open No. 2001-102944. Needless to say, however, thepulse position determining section may be configured in any othermanner.

[0022] In the mode calculating section 52, first, the modulation signalfed thereto is squared to form a squared value, and the amplitude of themodulation signal during a predetermined period is measured. Themeasured amplitude of the modulation signal is compared with apredetermined threshold value, and, if the amplitude remains lower thanthe threshold value during the predetermined period, the modulationsignal is recognized to be nearly in a no-sound state. If the modulationsignal is recognized not to be in a no-sound state, then the ratio ofthe high-frequency components filtered from the modulation signal by aHPF during the predetermined period to all the components of themodulation signal is calculated, and, if the calculated ratio is greaterthan a predetermined value, the modulation signal is judged to contain ahigh proportion of high-frequency components.

[0023] In this way, in the mode calculating section 52, first, whetheror not the input modulation signal is in a first mode, i.e., nearly in ano-sound state, is checked. Next, if the input modulation signal isfound not to be in the first mode, whether it is in a second mode, inwhich it contains a low proportion of high-frequency components, or in athird mode, in which it contains a high proportion of high-frequencycomponents, is checked. Thus, the mode calculating section 52distinguishes between three modes, namely the first to third modes.

[0024] Having distinguished between the three modes, the modecalculating section 52 notifies the interpolation width calculatingsection 53 of the recognized mode. The interpolation width calculatingsection 53 sets the interpolation width over which to cancel noise; thatis, it sets the interval of time over which, when pulse noise isdetected, interpolation is performed for waveform shaping aftercancellation of the pulse noise. Here, if the mode calculating section52 has recognized the first mode, the interpolation width is set to belongest, and, if the mode calculating section 52 has recognized thethird mode, the interpolation width is set to be shortest.

[0025] Here, since the modulation signal is fed in in the form ofsampled discrete signals, the interpolation width is set in terms of thenumber of data over which to perform interpolation. Specifically, in thefirst mode, the interpolation width encompasses ten data including theone at which the pulse noise was detected; in the second mode, theinterpolation width encompasses seven data including the one at whichthe pulse noise was detected; and, in the third mode, the interpolationwidth encompasses five data including the one at which the pulse noisewas detected.

[0026] Moreover, in the pulse noise reducing section 54, to remove thediscontinuity between the portion where interpolation was performed andthus pulse noise was cancelled and the remaining portion where nointerpolation was performed, the modulation signal is then subjected toprocessing with a LPF (low-pass filter). The cut-off frequency of theprocessing with the LPF here is set according to the mode. Specifically,the cut-off frequency is set to be lowest in the first mode and highestin the third mode.

[0027] Then, the data position at which the pulse noise is superimposedas detected by the pulse position determining section 51 and theinterpolation width set by the interpolation width calculating section53 at the data position at which the detected pulse noise issuperimposed are fed to the pulse noise reducing section 54. Then,linear interpolation is performed by using the data preceding andsucceeding the interpolation width, with the center of the interpolationwidth located at the data position at which the pulse noise issuperimposed, so as to determine the data at each data position withinthe interpolation width.

[0028] For example, suppose that, as shown in FIG. 3, pulse noise isdetected at a data position Y3, and that the third mode is recognized,with the result that the number of data within the interpolation widthis set to be five. Moreover, let the signal levels at the individualdata positions Y1 to Y5 within the interpolation width be y1 to y5, letthe signal level at the data position Xa immediately before theinterpolation width be xa, and let the signal level at the data positionXb immediately after the interpolation width be xb. Then, the signallevels y1 to y5 at the individual data positions Y1 to Y5 within theinterpolation width are set as follows:

y 1=(xb−xa)/6+xa

y 2=2×(xb−xa)/6+xa

y 3=3×(xb−xa)/6+xa

y 4=4×(xb−xa)/6+xa

y 5=5×(xb−xa)/6+xa

[0029]FIGS. 4A to 4C and FIGS. 5A to 5C show how this pulse cancellationis performed in a no-sound state and with a sinusoidal wave having afrequency of 3 kHz, respectively. FIGS. 4A and 5A show the modulatedsignal having pulse noise superimposed thereon, FIGS. 4B and 5B show themodulated signal after being subjected to interpolation with the numberof data within the interpolation width set to be five, and FIGS. 4C and5C show the modulated signal after being subjected to interpolation withthe number of data within the interpolation width set to be ten. FIGS.4A to 4C and FIGS. 5A to 5C are diagrams showing how interpolation isperformed differently with different interpolation widths.

[0030] When pulse noise is superimposed on the modulation signal in ano-sound state (in the first mode) as shown in FIG. 4A, if the number ofdata within the interpolation width is set to be five, the pulse noiseis not completely cancelled from the modulation signal afterinterpolation as shown in FIG. 4B, leaving behind an uncorrected part ofthe pulse noise. In this case, by increasing the number of data withinthe interpolation width to ten, it is possible to completely cancel thepulse noise and in addition restore the portion where the pulse noisewas cancelled and interpolation was performed to the no-sound state asshown in FIG. 4C.

[0031] On the other hand, when pulse noise is superimposed on themodulation signal when it is a sinusoidal wave having a frequency of 3kHz (in the third mode) as shown in FIG. 5A, if the number of datawithin the interpolation width is set to be ten, the modulation signalafter interpolation and pulse noise cancellation is output with adistorted waveform as shown in FIG. 5C. In this case, by reducing thenumber of data within the interpolation width to five, it is possible tocompletely cancel the pulse noise and in addition restore the portionwhere the pulse noise was cancelled and interpolation was performed toclose to a sinusoidal wave having a frequency of 3 kHz as shown in FIG.4C.

[0032] Thus, the higher the proportion of high-frequency components thatthe modulation signal contains is, the shorter the interpolation widthneeds to be set to be to achieve appropriate interpolation. In this way,in the pulse noise reducing section 54, pulse noise is cancelled andinterpolation is performed; then, through processing using an LPF, thediscontinuity between the interpolated and non-interpolated portions isremoved. As a result, the pulse noise reducing section 54 outputs themodulation signal with reduced pulse noise and with alleviateddistortion resulting from correction.

[0033] In this embodiment, the mode calculating section distinguishesbetween three modes, namely the first to third modes in which themodulation signal is in a no-sound state, contains a low proportion ofhigh-frequency components, and contains a high proportion ofhigh-frequency components, respectively. It is, however, also possibleto use a plurality of types of filter to more finely distinguish betweendifferent states of the modulation signal. In that case, by setting theoptimum interpolation width for each state, it is possible to alleviatethe distortion that occurs in the modulation signal as a result ofinterpolation. Interpolation may be achieved by any other method thanlinear interpolation, which is simple. Instead of converting themodulation signal into a digital signal in the detector section, it maybe converted into a digital signal after being converted to anintermediate frequency so as to be subjected to digital signalprocessing in the circuit blocks succeeding the IF stage.

Industrial Applicability

[0034] According to the present invention, it is possible to adjust,according to the state of an input signal, the interpolation width overwhich to perform interpolation. This makes it possible to performoptimum interpolation in different states. This helps alleviate thedistortion that occurs in the waveform of the input signal as a resultof interpolation and thereby obtain a natural waveform. In a no-soundstate or a state close thereto, by increasing the interpolation width,it is possible to prevent an uncorrected part of the superimposed noisefrom being left behind. On the other hand, when the proportion ofhigh-frequency components is high, by reducing the interpolation width,it is possible to alleviate the distortion in the waveform afterinterpolation. Moreover, by subjecting the input signal afterinterpolation to processing with an LPF, it is possible to alleviate thediscontinuity between the interpolated and non-interpolated portions.

1. A noise canceller including a pulse position determining section fordetecting pulse noise superimposed on an audio signal, the noisecanceller canceling from the input signal the pulse noise detected bythe pulse position determining section, comprising: a state calculatingsection that evaluates state of the audio signal; an interpolation widthcalculating section that, according to a proportion of high-frequencycomponents contained in the audio signal as evaluated by the statecalculating section, sets an interpolation width at a data position atwhich to cancel the pulse noise and perform interpolation; and a pulsenoise reducing section that processes data present within theinterpolation width set by the interpolation width calculating section,with a center of the interpolation width located at a data position atwhich the pulse position determining section has detected the pulsenoise from the audio signal, so as to cancel the pulse noise and performinterpolation and that then outputs the audio signal thus processed. 2.A noise canceller as claimed in claim 1, wherein, in the interpolationwidth calculating section, if the audio signal is judged to be nearly ina no-sound state by the state calculating section, the interpolationwidth is set to be longest, and, the higher a proportion ofhigh-frequency components that the audio signal is judged to contain bythe state calculating section is, the shorter the interpolation width isset to be.
 3. A noise canceller as claimed in claim 2, wherein, in thestate calculating section, first whether or not the audio signal is in ano-sound state is checked, and then whether or not the audio signalcontains a high proportion of high-frequency components is checked.
 4. Anoise canceller as claimed in claim 3, wherein, in the pulse noisereducing section, the audio signal having been subjected tointerpolation is further filtered with a low-pass filter.
 5. A noisecanceller as claimed in claim 4, wherein, in the interpolation widthcalculating section, the higher the proportion of high-frequencycomponents that the audio signal is judged to contain by the statecalculating section is, the higher a cut-off frequency of the low-passfilter is set to be before the audio signal is fed to the pulse noisereducing section.
 6. A noise canceller as claimed in claim 2, wherein,in the pulse noise reducing section, the audio signal having beensubjected to interpolation is further filtered with a low-pass filter.7. A noise canceller as claimed in claim 6, wherein, in theinterpolation width calculating section, the higher the proportion ofhigh-frequency components that the audio signal is judged to contain bythe state calculating section is, the higher a cut-off frequency of thelow-pass filter is set to be before the audio signal is fed to the pulsenoise reducing section.
 8. A noise canceller as claimed in claim 1,wherein, in the state calculating section, first whether or not theaudio signal is in a no-sound state is checked, and then whether or notthe audio signal contains a high proportion of high-frequency componentsis checked.
 9. A noise canceller as claimed in claim 8, wherein, in thepulse noise reducing section, the audio signal having been subjected tointerpolation is further filtered with a low-pass filter.
 10. A noisecanceller as claimed in claim 9, wherein, in the interpolation widthcalculating section, the higher a proportion of high-frequencycomponents that the audio signal is judged to contain by the statecalculating section is, the higher a cut-off frequency of the low-passfilter is set to be before the audio signal is fed to the pulse noisereducing section.
 11. A noise canceller as claimed in claim 1, wherein,in the pulse noise reducing section, the audio signal having beensubjected to interpolation is further filtered with a low-pass filter.12. A noise canceller as claimed in claim 11, wherein, in theinterpolation width calculating section, the higher a proportion ofhigh-frequency components that the audio signal is judged to contain bythe state calculating section is, the higher a cut-off frequency of thelow-pass filter is set to be before the audio signal is fed to the pulsenoise reducing section.